Supports a regex pattern in the contract and validate the data against it

Regex pattern should be supported in the contract as follows,

```
from pycontract import Contract, Types

my_contract = {
    'label': '^LABEL_[A-Z]*$',
    'name': Types.STRING
}
contract = Contract(my_contract)

assert contract.verify({'name': 'Product 1': 'label': 'LABEL_CREATED'}) is True # should result in true
```
